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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Mon, 24 Apr 2023 14:55:11 -0400
Message-ID: <CAEXW_YRfetnhgCw5OgnwhgZF_U+UkHN=uy=L8ovGLqn1UCtfTg@mail.gmail.com>
Subject: Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail
To:     Boqun Feng <boqun.feng@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, lance@osuosl.org,
        "Paul E. McKenney" <paulmck@kernel.org>
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This is amazing debugging Boqun, like a boss! One comment below:

> > > Or something simple I haven't thought of? :)
> >
> > At what points can r13 change?  Only when some particular functions are
> > called?
> >
>
> r13 is the local paca:
>
>         register struct paca_struct *local_paca asm("r13");
>
> , which is a pointer to percpu data.
>
> So if a task schedule from one CPU to anotehr CPU, the value gets
> changed.

It appears the whole issue, per your analysis, is that the stack
checking code in gcc should not cache or alias r13, and must read its
most up-to-date value during stack checking, as its value may have
changed during a migration to a new CPU.

Did I get that right?

IMO, even without a reproducer, gcc on PPC should just not do that,
that feels terribly broken for the kernel. I wonder what clang does,
I'll go poke around with compilerexplorer after lunch.

Adding +Peter Zijlstra as well to join the party as I have a feeling
he'll be interested. ;-)

thanks,

 - Joel